ISIS IR Profiles

[Keith Graham]

Hello,

When selecting a reference star that will be used for instrumental response and atmospheric correction, what is the most desirable star class to use? The Miles library shows different profile shapes for different classes, so it appears to me that the reference star should be very close to the target both in spectral class as well as proximity – is this correct?

That said, I am finding some inconsistencies in my IR profiles that have me questioning if I am performing the ISIS Continuum process correctly. I sometimes use the Automatic 2 correction while other times I perform the process manually using the slider. The automatic process appears to flatten out major bumps in the continuum whereas the manual process smooths the continuum through those major bumps while leaving them in place. So I am wondering when it is best to use the automatic process or the manual process. I tried attaching some of the IR profiles I have obtained for various star classes, but I get the message the the extension fit is not allowed (I am not sure why this is because that is the extension commonly used. I will be glad to attach them later if someone can please tell me what I need to do to allow their being attached). But I do find it interesting that the F9V and A2V profiles are very close while the others are quite different. Although these IR profiles are different, when I apply them in ISIS, the resulting stellar profiles are very close to the Miles profiles. Since this is my first attempt at low res spectroscopy, I am uncertain as to what I should expect with these IR profiles. Should they all be the same regardless of spectral class, or will the profiles be different for the different spectral classes?

Thanks,

Keith

[Robin Leadbeater]

Hi Keith,

There seems to be a lot of confusion around concerning this. There is no need to chose the same or similar spectral class for the reference star as the target. (We are effectively just using the ref star as a light source with a known spectrum). Hot stars (preferably fast rotating) are normally used as they have few lines to remove to get the good response shape when dividing by the reference spectrum and what lines are present are broadened by rotation. (In contrast cool stars are almost impossible to use as reference stars for this reason) The main problem when working in the IR is to get enough light from a hot star so you need to start with a bright reference. The other problem at low resolution is removal of tellurics of which there are many in the Red-IR. These blend into bands at low resolution so you need to generate template for your particular setup and scale it for the specific intensity of tellurics in your spectrum. You can use the reference star to do this, preferably by measuring it at low elevation. You can see a template I generated here for some Star Analyser measurements.
http://www.threehillsobservatory.co.uk/ ... poster.pdf

Cheers
Robin

[Robin Leadbeater]

Hello,

I tried attaching some of the IR profiles I have obtained for various star classes, but I get the message the the extension fit is not allowed (I am not sure why this is because that is the extension commonly used. I will be glad to attach them later if someone can please tell me what I need to do to allow their being attached).

Hi Keith,

You can usually get round the file type restrictions in forums by zipping them first. Not sure if it works here

Cheers
Robin

[Robin Leadbeater]

Hi Keith,

Sorry, I seem to have partly confused IR (instrument response) with IR (Infra-Red) All the above still applies though. (If we had to use the same type of star for instrument response as the target we would not be able to measure any unknown object ) I had a long debate about this with Richard Walker who originally insisted that the instrument response depended on the spectral type of the star measured, based on measurements he and others had made. I proved using Star Analyser spectra that this was not the case and he has included my results in the latest edition of his paper on processing spectra. In practise it is difficult to get a perfect measurement of instrument response over a wide spectral range using a slit spectrograph due to the various wavelength dependent selection effects of the slit, hence Christian's work on the wide photometric slit for Alpy

Note also that if you calculate the instrument response based on a star close to the target and use this directly to correct the target spectrum then this instrument response still contains the effect of the atmosphere and will vary with elevation and atmospheric conditions even for the same star (You would need to correct for atmospheric extinction to get a truly universal instrument response)

Cheers
Robin

[Keith Graham]

Hi Robin,

Thanks a bunch for the very informative response. I was especially interested in your experimentation that verified your suspicions that spectral class was not important for IR determination (yes, I do understand the confusion between the double use of “IR”. Perhaps we should change the abbreviation for instrument response to “RI”). And it is even more convincing that Richard Walker now agrees with your assessment based on your work.

It makes perfect sense to use a reference star close to the target in order to get the atmosphere correction. I could be assuming something here, but it seems to me that the A class stars appear to be the class of choice for response and atmosphere correction, and your explanation about fewer lines needing removal would be the reason for this choice.

So I would think, based on this conversation, that the best reference star for a target would be an A star close to the target. If no A star exists, then move to a G star if present. Perhaps a B star would be OK, but there are fewer of those, and some might even have emission lines (I do not know if that would be a problem).

Cheers,

Keith

[Robin Leadbeater]

Hi Keith,

Yes O B or A are the ones to use. Any cooler and the lines swamp the continuum in low/medium resolution spectra making it difficult to do the division accurately. You are right, there are not that many in the Miles database though so you can end up being some distance away.
The alternative is to use the list of bright A/B stars that Francois Teyssier generated, picking ones with low interstellar extinction E(B-V)
http://www.astronomie-amateur.fr/Docume ... inder.xlsm
but you then have to use a generic (eg Pickles) spectra for comparison which are not always guaranteed to be a good match.

That was why I suggested in our discussions with AAVSO that a nice project would be a sort of spectroscopic APASS, measuring Francois' list
using the Miles stars as references. The result would be a larger set of reliable bright A/B star spectra which could then be used as references for the simple "pick a nearby star as reference" approach. This could be done using ALPY with the new photometric slit for example but would need photometric conditions and the full rigourous correction for atmospheric extinction.

Robin

[Robin Leadbeater]

I forgot to add that reference stars should preferably be main sequence ie luminosity class V otherwise their spectra could be variable.

Cheers
Robin